Sustained activation of the human SII cortices by stimulus trains

To compare the functional properties of neurons in the human primary (SI) and secondary (SII) cortices, we recorded somatosensory-evoked fields (SEFs) from seven healthy subjects to single electric stimuli and stimulus trains delivered to the median nerve at 8--12 Hz. The SI and SII cortices responded strikingly differently to stimulus trains: whereas SI followed each stimulus with a sharp transient response up to at least 12 Hz, the transient responses were much less prominent at SII, which mainly responded with a sustained field that returned to base level at 800--1000 ms. The different response patterns of SI and SII suggest that the inhibition, following the early excitatory responses, is weaker at SII than SI, or that inhibitory responses of these two areas differ in their relative timing.